Hi SleepPixie,
I had my assessment at the start of May, and am now about to finish my course and go on line in about 10 days. The assessment day starts off with you sitting in a crew seat and putting on a life jacket (I guess to exclude the more 'horizontally challenged' applicants), and then you'll be parked in the canteen. It's a good idea to spend this time getting to know the other applicants, as it'll show when you do the group exercises later on. You'll then go into a company presentation, and a video about the life of cabin crew, a maths test (adding up various items using an in flight sales menu, working out change in various currencies etc)and a comprehension test (reading some text and answering questions. You'll then go into groups and do a couple of activities (they vary, but we built a house out of lego (testing how you work as a team) and role played complaining customers / managers).
Then a break while they decide who stays and who goes, and then lunch. You'll come back in the afternoon for a 2 on 1 interview which lasts 45 min to an hour.
You should be fine with the degree certificate, I got the impression that it would remove the need for A/O level certificates. It mght be worth giving the recruitment guys a call or an email to confirm, they're all really friendly in the office.
Best of luck!!